1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a grid for a fuel assembly made from zirconium alloy metal material having a predetermined grain orientation which reacts to irradiation so as to offset the effects of irradiation-induced stress relaxation of the grid components which hold the fuel rods of the assembly.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids as well known in the art are used to precisely maintain the spacing between the fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally retain the rods against longitudinal movement. Conventional designs of grids include a multiplicity of interleaved straps having an egg-crate configuration designed to form cells which individually accept the fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and relatively rigid protrustions (called dimples) formed into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell. Additionally, outer straps are attached together and peripherally enclose the grid straps to impart strength and rigidity to the grid.
In view that the operation of the reactor core involves irradiation of the fuel rods and that the grids are irradiated along with the fuel rods they support, the grids inherently degrade somewhat over time as a result. In particular, notwithstanding improvements in grid construction, such as illustrated and described in U.S. Pat. No. 4,474,730 to Hellman et al and assigned to the assignee of the present invention, over the life of the fuel assembly the metal material of the interleaved straps forming the grid, and thereby the springs and dimples formed therein, is subject to stress relaxation due to irradiation. This effect is especially marked in the case of grids fabricated from zirconium alloy metal material. The occurrence of stress relaxation of the springs, dimples and interleaved straps of the grid causes a general loosening of the grid's hold on the fuel rods, introducing gaps between the fuel rods and the grid and decreasing the supporting forces imposed by the grid on the fuel rods. Such loosening results in excessive fretting of the fuel rods and lengthwise distortion or bowing thereof over time.
Consequently, a need exists for a technique to compensate for irradiation-induced stress relaxation of the metal material of grids, especially those fabricated from zirconium alloy metal material. The technique must be one which reduces or eliminates the deleterious effects of stress relaxation on the fuel rods being held by the grids.